Gas turbine engines are typically employed to power aircraft. Typically a gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core.
The engine core comprises a compressor and a turbine, each of which have a plurality of rows of radially extending aerofoil members in the form of blades and stators. The aerofoils use surface curvature to change the static pressure of flow and thus provide lift. Such aerofoil rows suffer from secondary flows that arise on endwalls of the aerofoils and produce losses.
Friction on annular walls of flow passages (defined between two adjacent aerofoils) creates a boundary layer of slower moving air. As this air passes between the aerofoils it is more strongly influenced by the pressure gradient between the lower and upper surfaces of adjacent aerofoils, whereas the faster air outside the boundary layer is in equilibrium with the pressure gradient and is turned to the design exit flow angle. The slow boundary layer airflow may be over-turned (that is, turned further than the design angle) and rolled up into vortices, creating secondary flows that result in aerodynamic losses.
In compressors the problems associated with these secondary flows are exacerbated because generally compressor aerofoil rows diffuse the flow. Typically, the over-turned boundary layer will not simply roll up into a vortex, but additionally a region of separated flow will form in the corner between the aerofoil suction surface and the endwall. In parts of this separated region, the airflow may be reversed.
This corner separation is a source of significant losses, typically larger than the losses arising from “standard secondary flows” (such as in turbines). Corner separation may also cause significant blockage of the flow, reducing the mass flow delivered by the compressor. Furthermore, the presence of the secondary flow and corner separation causes the flow angle at exit from the row to deviate from the design angle. Consequently, the incidence angle of the flow onto the next aerofoil row deviates from its design angle, reducing that row's aerodynamic efficiency.